The present invention relates to electric motor starter circuits for operating motors which cannot be restarted under load, such as motors for operating rock crushers.
Large AC electric motors are typically energized through a heavy duty electromagnetic control relay commonly known as a motor starter. Often, momentary pushbutton Start and Stop control is provided. In a typical pushbutton Start switch arrangement, the actuating coil of the power relay is initially energized through normally-open contacts of the momentary pushbutton Start switch. Thereafter, in what is commonly know as latch-on or holding circuit arrangement, normally-open auxiliary contacts of the power relay or other control relay bypass the Start switch contacts to maintain energization to the start relay coil so long as AC supply voltage is available. The Stop switch is a momentary pushbutton switch having normally-closed contacts connected in series with the power relay coil so that the holding circuit is broken when the Stop switch is actuated. When this occurs, the power relay drops out, dropping the motor off line.
It will be apparent that such a magnetic motor starter is susceptible to unintended drop out upon the occurrence of momentary interruptions in the AC power supply as sometimes occur as a result of such events as lightening strikes momentarily tripping circuit breakers, utility company switching operations, or the like. Even a momentary drop out of the control relay causes the holding circuit to be broken. For many AC motor operated loads this poses only a minor inconvenience, as the Start switch may simply again be actuated so as to resume operation.
However, there are other electic motor and load combinations which cannot be restarted under load, and significant equipment down time may be required to prepare the motor and load for restarting. One example of such a load with which the present invention is particularly concerned is a rock crusher wherein rocks are fed between a pair of confronting jaws which reciprocate relative to each other. A typical rotary-to-reciprocal motion transmission arrangement for a rock crusher deliberately has a fairly large inertial mass, for example in the form of a flywheel, which stores energy during each return stroke of the jaws such that, during the forward or crushing stroke of the jaws, additional force in excess of that which could be provided by the motor alone is available. In normal operation, a rock crusher is started without any rocks between the jaws, and rocks are not fed until such time as the rock crusher attains operating speed and momentum has accordingly built up in the inertial mass. Similarly, when it is desired to turn off the equipment, the supply of rocks to be crushed is terminated, and the rock crusher is turned off only after the jaws are cleared.
However, during momentary power outages as described above which cause a magnetic motor starter to drop out removing the motor from the line, the rock crusher is sometimes unnecessarily stopped because the motor drops off line while the rock crusher still has sufficient momentum to continue operation upon resumption of the power supply. Once unintentionally stopped in this manner, the rock crusher cannot be restarted until the jaws are cleared, resulting in an unacceptable down time and the loss of production.
By the present invention, there is provided a circuit arrangement for preventing motor starter relay drop out during momentary interruptions of the main AC power source, during which interruptions the rock crusher maintains sufficient momentum to resume normal operation upon restoration of the main AC power supply. It should be noted, however, upon the occurrance of longer term power failures there is no choice but to clear the rocks between the jaws before resuming opration. Upon the occurrance of such longer-term power interruptions, the circuit arrangement of the present invention allows the motor starter relay to drop out.